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Rheology of Protein-Stabilised Emulsion Gels Envisioned As Composite Networks. 2 - Framework for the Study of Emulsion Gels

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Rheology of Protein-Stabilised Emulsion Gels Envisioned As Composite Networks. 2 - Framework for the Study of Emulsion Gels Rheology of protein-stabilised emulsion gels envisioned as composite networks. 2 - Framework for the study of emulsion gels Marion Roulleta,b,∗, Paul S. Cleggb, William J. Fritha aUnilever R&D Colworth, Sharnbrook, Bedford, MK44 1LQ, UK bSchool of Physics and Astronomy, University of Edinburgh, Peter Guthrie Tait Road, Edinburgh, EH9 3FD, UK Abstract Hypothesis The aggregation of protein-stabilised emulsions leads to the formation of emulsion gels. These soft solids may be envisioned as droplet-filled matrices. Here however, it is assumed that protein-coated sub-micron droplets con- tribute to the network formation in a similar way to proteins. Emulsion gels are thus envisioned as composite networks made of proteins and droplets. Experiments Emulsion gels with a wide range of composition are prepared and their viscoelasticity and frequency dependence are measured. Their rheological behaviours are then analysed and compared with the properties of pure gels presented in the first part of this study. Findings When the concentrations of droplets and protein are expressed as an effective volume fraction, the rheological behaviour of emulsion gels is shown to depend mostly on the total volume fraction, while the composition of the arXiv:2007.11843v2 [cond-mat.soft] 5 Oct 2020 gel indicates its level of similarity with either pure droplet gels or pure protein ∗Current address: BioTeam/ECPM-ICPEES, UMR CNRS 7515, Universit´ede Stras- bourg, 25 rue Becquerel, 67087 Strasbourg Cedex 2, France Email addresses: [email protected] (Marion Roullet), [email protected] (Paul S. Clegg), [email protected] (William J. Frith) Preprint submitted to Journal of Colloid and Interface Science October 6, 2020 gels. These results help to form an emerging picture of protein-stabilised emulsion gel as intermediate between droplet and protein gels. This justifies a posteriori the hypothesis of composite networks, and opens the road for the formulation of emulsion gels with fine-tuned rheology. Keywords: Colloidal gel, Rheology, Emulsion, Sodium caseinate, Viscoelasticity, Protein-stabilised droplet, Formulation, Mixture Graphical abstract 1. Introduction Emulsion gels are materials of great interest, because of their many appli- cations in foods, drug-release pharmaceutical products, and novel personal care products [1, 2, 3]. Emulsion gels are soft solids that contain a liquid phase, usually water, trapped within the pores of a network comprised of emulsion droplets [4]. However, this general description conceals the very different structures that emulsion gels can have, depending on their compo- sition [5]. Despite the increased efforts in relating the mechanical properties of emulsion gels to their composition, the full understanding of these links is still lacking. Traditionally, for emulsion gels, the distinction is made between emulsion- filled gels - in which the droplets act as fillers in a viscoelastic gel matrix - and emulsion particulate gels - in which aggregated droplets form a gel network of 2 their own [4, 5]. Emulsion-filled gels have been studied widely, and a mean field theoretical approach has been used to model the gel matrix, that is often a protein gel, as a continuous medium [6, 7]. In that framework, the emulsion droplets are elastic inclusions that can be deformed upon stressing the emulsion gel [8], and that present interactions with the matrix that are either attractive (active fillers) or repulsive (passive fillers) [9, 10, 11, 12, 13, 14]. Emulsion particulate gels have attracted less attention, and they were considered to be similar to other colloidal gels [15, 16, 17]. An exception is the first part of this series, in which pure gels made of protein-stabilised emulsion droplets have been studied and their rheological properties characterised [18]. At this point, it is useful to note the difference between emulsion gels and concentrated emulsions like mayonnaise, which also display a solid-like behaviour. Emulsion gels present a solid gel network, that can be relatively dilute, and that traps a significant amount of solvent within its pores. By contrast, concentrated emulsions are made of jammed repulsive droplets, that are limited in their mobility by the presence of the other droplets [19]. Such jammed systems are often refered to as colloidal glasses [20], and are comparable to other glasses made of soft particles, such as star polymers and microgels [21, 22, 23, 24]. The present study will focus on emulsion gels, and the emulsions used to prepare the gels will thus be kept at concentrations for which a low-shear viscosity can be defined. The binary distinction between emulsion-filled gels and emulsion partic- ulate gels is however limited by the strong assumptions that are made when defining these two situations. First, the approximation of a continuous ma- trix, in which the droplets are embedded, does not always apply. Indeed, this matrix is often a protein gel, which is a ramified network with a mesh size of a few microns [25, 26], so it is assumed that the droplets are much larger. Yet, sub-micron droplets are widely used in commercial products, as their production is made easier by the advances in emulsification techniques, and notably the use of microfluidizers [27, 28]. There can thus be emulsion gels in which the droplets are smaller than the pores of the matrix, which cannot then be approximated by a continuous medium. Furthermore, it is worth noting that the formation of large aggregates and networks of attractive droplets would make a significant contribution to the overall viscoelasticity of the emulsion gel, but this is generally not considered in the emulsion-filled gel model, while it is central to the existence of emulsion particulate gels. Previous efforts to account for droplet aggregation, and its contribution to the viscoelasticity, in the emulsion-filled gel model, have not 3 yet lead to an accurate estimation of the changes in viscoelasticity induced by droplet aggregation [29]. It thus appears necessary to fill the gap between these two models of emulsion gels, to define a more accurate framework for the study of these materials. This study focuses on the gels produced by destabilising protein-stabilised emulsions, in which proteins - more specifically sodium caseinate - act both as surface-active emulsifier, to form sub-micron oil droplets stabilised by steric and electrostatic repulsion at neutral pH, and as gelling agent. When the emulsion is acidified, the electrostatic repulsion is decreased, and at the protein isoelectric point, attractive van der Waals interactions lead to the gelation of the proteins and of the protein-coated droplets [7]. In order to ensure a sufficient surface coverage of the droplets in real systems, and thus a good stability of protein-stabilised emulsions, it is common to work with a protein excess, so a mixture of protein-coated droplets and of unadsorbed proteins is obtained after emulsification [30]. In summary, the gels studied here are made of sub-micron droplets covered with proteins, and of sodium caseinate, structured into self-assembling aggregates of around 20 nm. The oil droplets are part of the network, as they exhibit an attractive interaction between them mediated by the adsorbed proteins at their interface. The protein-stabilised droplets and caseinate assemblies presented here have been thoroughly characterised in a previous study [31]. In the first paper of this pair, pure gels of caseinate assemblies and pure gels of caseinate-stabilised sub-micron droplets were prepared and charac- terised [18]. It was shown that the gelation of protein suspensions and of purified suspensions of droplets led to gel networks with a characteristic length-scale of the order of a few microns. The emulsions studied here are thus characterised by droplets that are smaller than the length-scale of the networks, and these droplets aggregate extensively to form a space-spanning fractal network, even at low concentration. In addition, it was shown that the concentrations of the suspensions of proteins, and of protein-stabilised droplets, could be scaled by the effective volume fraction φeff , and their viscosity could be analysed in the framework developed for soft colloids [31]. This parameter φeff represents the volume occupied by the particles in the sample divided by the total volume. It is calculated by multiplying the concentration by a parameter k0, derived by approximating protein aggregates and protein-stabilised droplets to model hard spheres when in semi-dilute suspensions. This same framework was used to study the gels formed by the two types of suspensions in the first 4 part of this series, and the scaling by the effective volume fraction φeff made it possible to emphasise the similarities between the two types of gels at fixed φeff , both in terms of microstructural features and of rheological properties [18]. The present work envisions protein-stabilised emulsion gels as compos- ite networks made of un-adsorbed protein assemblies and protein-coated droplets. This approach relies on the hypothesis that there is little distinc- tion between droplets and un-adsorbed proteins in the way each contributes to the properties of the gel of mixture. This is because the most relevant length-scale to study the rheological and microstructural features of colloidal gels appears to be the length-scale of strands of particles [32, 33, 34, 35]. Con- sequently, the systems are examined at a much larger scale than of the single particles, and the discrepancy in size and structure of the protein aggregates and protein-stabilised droplets is thus assumed not to be critical. Here, protein-stabilised emulsion gels with a wide range of protein and droplet content are prepared, and their rheological properties are charac- terised and analysed as a function of the composition of the sample. The emulsion gels are then compared to the pure gels of proteins and droplets, to identify the contribution of each of the components to the rheological prop- erties of the composite networks.
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